In the manufacture of integrated circuits, wafer surface planarity is of extreme importance. Chemical mechanical polishing (CMP) is being increasingly used for dielectric planarization and metal polishing processing steps.
Referring to FIG. 1, in a conventional CMP system 100, a wafer 102 whose top surface 104 is to be polished is held in an inverted position by a carrier assembly 106 and polishing arm 108. The wafer 102 is held in a position against a rotating polishing pad 110 which removes material from the top surface 104 of the wafer 102 from mechanical abrasion from the polishing pad 110 and particles in the slurry dispensed onto the polishing pad 110 through a slurry inlet 128, and from chemical action from the slurry on the polishing pad. Rotation of the polishing pad during the polishing process is caused by a motor 112, while rotation of the wafer is caused by another motor 114. In addition there is a periodic translation motion by the polishing arm 108 so as to use different portions of the polishing pad over time.
The carrier assembly 106 is a pneumatic carrier that is connected to a vacuum pump and air pressure pump assembly 120 via tubing (not shown) in the polishing arm 108. The pneumatic carrier includes a perforated steel plate 122 and a capture ring 124, which together hold a perforated carrier pad 126 and the wafer 102. When a wafer is first moved by an automated wafer transportation system 130 from a wafer transport tray 132 to the carrier assembly 106, a vacuum pump is coupled to the carrier assembly 106 so as to hold the wafer in place while the polishing arm moves the wafer into position adjacent the polishing pad 110. After the wafer is in position and polishing begins, backside pressure can be applied to the wafer by a downward movement of the polishing arm 108 and application of positive air pressure through the perforated metal carrier plate 122 and carrier pad 126.
Some difficult problems with the CMP process in the commercial manufacturing environment are maintaining a stable material removal rate and maintaining uniformity of material removal on each wafer when processing hundreds of wafers with a single polishing pad. One of the key parameters in improving uniformity is the control of backside pressure (sometimes called backpressure) and the distribution of backside pressure. Backside pressure is the pressure between the surface of the wafer to be polished and the polishing pad. Although a user of the CMP system can control the pressure exerted on the wafer, the user cannot adjust the distribution of the backside pressure.
In the prior art, the distribution of the backside pressure is fixed and is dependent on the perforations of the carrier plate 122 and the carrier pad 126. Conventionally, equipment vendors design the distribution of backside pressure through using dummy wafers (i.e., non-pattern wafers) and placing the perforations accordingly. To vary the distribution of the backside pressure, the user has to order a new carrier plate and carrier pad. The user must wait for the equipment vendor to design, manufacture and ship the new carrier plate and carrier pad. This can be costly and time-consuming as it adds to the down time of the CMP system.
Therefore, it would be advantageous to have a system and method for readily adjusting the distribution of the backside pressure.